The present invention relates to semiconductor processing and, more particularly, to thermal reactors used for epitaxial deposition and processes with similar requirements. A major objective of the present invention is to provide for improved gas injection for a barrel reactor to enhance epitaxial deposition uniformity.
Much of recent technological progress is identified with increasingly miniature integrated circuits which have been providing greater functional density at lower costs. Semiconductor processing technology has provided for increased functionality, in part, by permitting smaller features to be patterned onto a semiconductor wafer, and has provided for lower costs, in part, by developing equipment which can handle larger wafers, thereby increasing the number of circuits that can be made together. Fabricating smaller features requires thinner layers to be deposited and observance of the stricter tolerances they require. These stricter tolerances place increased demands of deposition uniformity across a wafer. Since larger wafers are being used, this greater uniformity must be attained over larger areas.
Epitaxial deposition is the deposition of a single crystal layer on a substrate such that the crystal structure of the layer is an extension of the crystal structure of the substrate. Epitaxial deposition permits the fabrication of bipolar integrated circuits and other devices with higher switching speeds, breakdown voltages and current handling ability. Most commonly, a silicon epitaxial layer is deposited on a silicon substrate, but different substrate materials can be used and the epitaxial layer need not be of the same material as the substrate.
Epitaxial deposition involves exposing a substrate to appropriate reactant gases under carefully controlled conditions including elevated temperatures and sub-ambient pressures. Typically, the substrate is etched, e.g., using hydrogen chloride gas, to prepare nucleation sites prior to deposition. A silicon epitaxial layer can then be deposited using hydrogen reduction of silicon tetrachloride (SiCl4) or pyrolysis of silane (SiH4). Other reactant gases are used less commonly, e.g., trichlorosilane (SiHCl3) and dichlorosilane (SiHCl2). To date, the foregoing vapor deposition techniques have yielded higher quality epitaxial layers than have been attained using sputtering or evaporation techniques.
Three thermal reactor designs are commonly used for epitaxial deposition, namely, vertical reactors, horizontal reactors and barrel reactors. In a vertical reactor, reactant gases enter and spent gases exit a bell-jar chamber vertically through its bottom. The gases swirl within the chamber to react at the surface wafers supported by a horizontally-oriented disk-shaped susceptor. The susceptor, in this and in the other designs, serves to convert energy from a source external to the reaction chamber to the heat require to promote the desired deposition reaction at the wafer. The susceptor rotates, helping to maintain a uniform temperature on the susceptor and to distribute reactant gases throughout the chamber.
In a horizontal reactor, reactant gases enter at one end of a horizontally extending tube and are exhausted out the other end. Both rotating and non-rotating susceptors can be used. The susceptor can be inclined a few degrees from the horizontal so that used gas is introduced at every wafer position.
A barrel reactor combines features of both the horizontal and vertical designs. A susceptor with faces slightly inclined relative to a vertical axis rotates within a bell-jar chamber. Wafers are placed in vertical rows on each susceptor face while reactant gas, introduced at the top of the chamber, flows downwardly over the wafers and is exhausted from the chamber bottom. The barrel reactor is generally favored because it permits precise depositions on a large number of wafers at once. Along with the advantage of greater wafer capacity comes the challenge of ensuring deposition uniformity from wafer to wafer, as well as uniformity across individual wafers.
Deposition uniformity in a barrel reactor is sensitive to the reactant gas flow pattern, which is in turn dependent on the way that the gases are introduced into the chamber. In one approach to gas injection for a barrel reactor, a pair of nozzles are disposed near the top of the chamber and symmetrically relative to a plane which bisects the chamber vertically. The nozzles are oriented so that their respective jets collide, resulting in a combined gas flow which is directed primarily downward through the chamber and over the vertically supported wafers.
Since chamber gas flow at the wafer results from the collision of gas jets, it is apparent that precise control over the balance and orientation of the gas jets is critical to deposition uniformity. If one jet is stronger, a lateral or spiral component to the gas flow could result, impairing deposition uniformity. Similar problems occur if the nozzles are not oriented symmetrically. Where the jets are balanced and the nozzle are oriented symmetrically, consideration must be given to the precise orientations of the nozzles to ensure deposition uniformity. Thus, the elevation and azimutal position of the nozzles must be determined. The correct nozzle orientations can vary according to gas flow rate, thermal conditions, and the reaction rate for the selected deposition reaction.
In an illustrative barrel reactor, each nozzle includes a tube which extends from a spherical element which serves as a ball joint. During normal operation, the ball joint is clamped between a metal ring and a front wall of a tubular housing for the nozzle. The metal ring is held in place by a set screw which can be loosed to permit adjustment of the nozzle orientation. Proper alignment can be achieved by inserting a Cartesian grid near the target intersection of the shroud and bisecting plane. A needle can be inserted into each nozzle and the distal end of the needle can be aligned with a selected point on the grid. Once the desired orientation is attained, the set screw is tightened and the needle removed. This procedure is then repeated for the other nozzle. The nozzles are coupled through similar conduits and valves to a common gas source. The valves can be adjusted to obtain the desired jet flow balance.
While the illustrative barrel reactor has been very successful from commercial and technical perspectives, refinements are required to accommodate the stricter tolerances that are sure to be imposed by advancing technology and market demands. The described nozzle alignment approach is somewhat inconvenient and unreliable since it generally requires some disassembly, e.g., removal of the gas lines connected to the chamber, to be implemented. The adjustment mechanisms are cumbersome in that a change in a reaction parameter can require disassembly and ad hoc adjustments to each nozzle and each valve, with each adjustment potentially requiring further adjustments for the other valve or nozzle. Thus, determining the correct nozzle orientations and valve positions can be time consuming. Finally, even when the proper jet strengths and nozzle orientations are achieved, there can still be problems obtaining the desired level of deposition uniformity.
What is needed in a gas injection system for a barrel-design epitaxial reactor which provides for a reactant gas flow with better promotes both single-wafer and inter-wafer deposition uniformity. Preferably, such a system would provide for more convenient and precise nozzle orientation adjustment and gas jet rate adjustments.